Two of three undefined components of TYI-S-33, a standard medium used for axenic cultivation of Entamoeba histolytica, have been replaced with defined nutrients. Minimal requirements of the parasite for nucleic acid precursors and B vitamins have been determined using the semi-defined medium. Strains of T-cell deficient and macrophage deficient mice have been screened as potential models for the study of parasite clearance in Giardia lamblia infections. No correlation has been found between ability to clear the parasite and specific immune deficiencies. A new hamster model using animals less than a week old has been developed to study the influence of iron on the production of hepatic lesions in amebiasis. The young animals are more sensitive to infection than the formeraly employed adult animals and can be inoculated percutaneously, obviating the need for a laparotomy. Freeze-preservation of a variety of lumen dwelling protozoa in liquid nitrogen has been facilitated through refinement of controlled-rate and non-controlled-rate freezing techniques.